STRATOSE THALLUS 97 



thallus with which the lower cannot keep pace 1 . A little way back from 

 the edge, the result of the stretching is seen in the splitting asunder of the 

 felted hyphae of the under surface, and in the consequent formation of a 

 reticulate series of ridges known as the veins or nerves ; they represent the 

 original tomentose covering, and are white, black or brown, according to the 

 colour of the tomentum itself. The naked ellipsoid interstices show the 

 white medulla, and, if the veins are wide, the colourless areas are correspond- 

 ingly small. Rhizinae are formed on the nerves in several of the species, 

 and anchor the thallus to the support. In Peltigera canina, the under surface 

 is almost wholly colourless, the veins are very prominent (Fig. 55), and are 

 further strengthened by the growth and branching of the parallel hyphae of 

 which they are composed. They serve to strengthen the large and flabby 

 thallus and form a rigid base for the long rhizinae by which the lichen clings 

 to the grass or moss over which it grows. 



The most perfect development of strengthening nerves is to be found in 

 Hydrothyria venosa*, a rather rare water lichen that occurs in the streams of 

 North America. It consists of fan-like lobes of thin structure, the cortex 

 being only about one cell thick. The fronds are about 3 cm. wide and they 

 are contracted below into a stalk which serves to attach the plant to the 

 substratum. Several fronds may grow together in a dense tuft, the expanded 

 upper portion floating freely in the water. Frequently the plants form a 

 dense growth over the rocky beds of the stream. 



At the point where the stalk expands into the free erect frond, there 

 arise a series of stout veins which spread upwards and outwards. They are 

 definitely formed structures and not adaptations of pre-existing tissues : 

 certain hyphae arise from the medulla at the contracted base of the frond, 

 take a radial direction and, by increase, become developed into firm strands. 

 The individual hyphae also increase in size, and the swelling of the nerve 

 gives rise to a ridge prominent on both surfaces. They seldom anastomose 

 at first but towards the tips they become smaller and spread out in delicate 

 ramifications which unite at various points. There is no doubt, as Bitter 1 

 points out, that the nerves function as strengthening tissues and preserve the 

 frond from the strain of the water currents which would, otherwise, tear apart 

 the delicate texture. 



1 Bitter 1899. 2 Sturgis 1890. 



S. L. 



